The basic concepts of passive immunization and the potential of antibody therapy to confer immunity against infectious diseases were introduced already in the late 19th century. This approach was also later implemented to extensively treat and prevent infections, but with the development of effective vaccines, it became restricted to only a few medical conditions such as snake bites, neutralization of toxins, and prevention of rabies infection. This has dramatically changed in the last decade, as antibodies have been widely used in the clinic for the treatment of COVID-19 and the prevention of respiratory syncytial virus (RSV) infections. A stepping-stone for the progress in monoclonal antibody generation was the development of single-cell antibody cloning techniques that made it possible to develop effective neutralizing antibodies against highly mutable viruses such as influenza virus and HIV-1. Here, we review the use of passive immunotherapy in the clinic for treating and controlling SARS-CoV-2 and RSV infections. We further discuss key developments that have made it possible to use monoclonal antibodies against the highly mutable HIV-1 and influenza virus and advanced clinical trials designed to evaluate the efficacy of such an approach. Finally, we present recent findings that demonstrate that passive immunization can elicit long-term immunity in the host.